The invention relates to data storage devices that record data in a series of closely spaced data tracks and in particular, to a method for calibrating a tape media using staggered calibration tracks.
It is a long standing problem in the field of digital tape drives to align the tape head to avoid recording data on a previously written data track while also providing an increased number of data tracks on the length of tape media. Data storage on tape media typically involves recording flux transitions in a series of narrow data tracks which are spaced closely together to maximize the amount of data that can be stored in a given length of tape media. During the recording process, the tape media is moved past the tape head as flux transitions are imparted in a thin line referred to as a data track.
For linear recording tape devices, the data tracks have definite starting and stopping points. Normally, tape media motion continues in a straight line until the end of the tape media is reached and the tape media is stopped. This motion is referred to a single pass and for purposes of discussion will be defined to be in the horizontal direction. Motion perpendicular to a recorded data track will be defined to be in the vertical direction. For the class of tape drives under discussion, the tape head is held still at a fixed vertical position during each pass while the tape media travels between a supply reel and a take up reel.
A tape head assembly often has multiple magnetic gaps allowing it to record more than one data track simultaneously. In early designs it was common for a multi-gap tape head to write a limited number of relatively wide data tracks covering the full width of the tape media in a single pass. Some blank space was left between the data tracks to reduce interference from adjacent data tracks. The vertical distance between adjacent data tracks was largely dependent on the tolerances of the tape head itself which was fixed in a single position.
Improvements in recording technology have made it possible to read and write very narrow data tracks, provided the read and write heads in the tape head are spaced much further apart than the width of a single data track. This requirement necessitated a new technique to write data tracks spaced closely or even directly adjacent to one another. One common method is to move the tape head perpendicular to the direction of tape media travel. Before a recording pass, the tape head is moved by an amount equal to one data track width and then held in that position for a complete pass of the tape media. Subsequently, tape media movement is stopped and restarted in the opposite direction, and writing begins again with one or more write heads positioned near the previously recorded data track or tracks. In this way, a large number of data tracks covering most of the tape media surface can be recorded by a small number of write heads, at the expense of a making a mechanism to move the tape head.
When the end of the tape media is reached, the tape head position is changed and the tape media must be stopped and then restarted in the opposite direction. The time required at the end of a pass to decelerate, stop and accelerate the tape media in the opposite direction represents an undesired interruption of the recording or playback process. To avoid additional delays from rewinding the tape, it is common to simply reverse the direction of reading or writing once the end of tape media is reached. This method is known in the art as xe2x80x9cserpentinexe2x80x9d recording in view of the shape of the back and forth path traveled by the tape head relative to the tape media. In other words, data is written or read left to right during one pass and right to left during the next pass.
The ability to accurately locate and vertically position the tape head before each pass is of great importance. If the tape head position is incorrect, it is possible to record new data tracks partially or even completely overlapping previously recorded data tracks. In this case, reproducing the older data becomes difficult or impossible. Conversely, if the data tracks are too far apart the tape media capacity is wasted.
All tape head motions must be made relative to a known reference point. One early scheme used the physical end of travel of the moving tape head mechanism as a starting or xe2x80x9czeroxe2x80x9d point. This scheme worked well as long as the tolerances between the tape head bracket assembly, the base plate and the tape guides were all well known and constant.
An improved system, taught by U.S. Pat. No. 4,476,503 Solhjell, uses the edge of tape media as a reference point. The tape drive automatically locates the edge relative to one of the read heads in the tape head. This is done by first moving the tape head so low that both a read and write head, which are in line with each other, are positioned below the edge of the tape media. The read head is turned on at the same time that tape media is moved and the tape head vertically upward until the read head detects a signal from the tape media. This method eliminates several sources of positioning errors although the tape head must still move in imprecise increments from the reference point at the edge of the tape media to the next data track. However, a problem occurs due to wear on the edge of the tape media as well as imprecision finding that edge, which will contribute to an overall position error.
A further improvement in data track locating systems uses one or more calibration or reference tracks written for a short distance at the beginning of the tape media. A solution used by Quantum Corporation of Milpitus, Calif. uses two forward and two reverse calibration tracks written in an otherwise blank section of tape media known as the calibration area. The two forward calibration tracks are written simultaneously by separate write heads in the tape head assembly while the tape media is moved in the forward direction through the calibration area. The two reverse calibration tracks are likewise written in the opposite direction in the same horizontal area, but at a different vertical location to avoid interference with the forward calibration tracks.
Typically, the calibration area is located at the beginning of the tape and uses only a small fraction of the tape media length. Once calibration tracks are written on the tape media, the calibration tracks are not rewritten unless the entire tape media is erased. If additional data needs to be recorded on a partially recorded tape media, the calibration tracks are first located and used as a starting reference point to locate the next data track of interest. This method eliminates the problem of tape edge wear and reduces the distance the tape head must move vertically from the measured calibration tracks to the next data track of interest.
Once the calibration tracks are located, all forward data tracks are written at fixed vertical offsets relative to the forward calibration tracks and all reverse data tracks are likewise written at fixed vertical offsets relative to the reverse calibration tracks. This is necessary because tape media position shifts in the tape guides with changes in tape media direction. A problem with this solution is that it requires that the tape head be capable of moving a precise distance relative to the calibration tracks, thus using a tape head that is costly.
An alternative system described by U.S. patent application Ser. No. 09/876,705 proposes to write each calibration track as each data tracks is written instead of in a separate, prior operation. Each calibration track is essentially a preamble to each data track, and is not written until the tape drive is ready to write the data for that data track. After the preamble is written, the data track begins without any interruption of the tape motion or repositioning of the head assembly. While this system has numerous advantages there are also some limitations. For instance, because the tape may be removed from the drive before it is completely filled and then placed in another tape drive, subsequent calibration tracks may be written by a different tape drive. This results in undesired additional errors in the system as the tolerances between the two tape drives will vary.
For these reasons a need exists for tape media calibration that provides improved data track positioning by providing a plurality of calibration tracks corresponding to a plurality of data tracks without increasing the time required to for the tape dive to record the plurality of calibration tracks.
The present method for calibrating a tape media using staggered calibration tracks provides increases the accuracy in aligning the tape head prior to reading or writing data to or from a data track. The staggered calibration track comprises a plurality of successive calibration tracks wherein each calibration track corresponds to one or more data tracks. The staggered calibration track is written by positioning the write head to a predetermined location, then incrementally adjusting the vertical position of the write head as the tape media is moved horizontally past the write head in a continuous manner. A forward staggered calibration track and a reverse staggered calibration track are written for use in aligning the tape head for reading or writing data to or from the tape media in the forward and reverse directions. Only one stop/start motion of the tape media is required after the forward calibration tracks are written and before the reverse calibration tracks are written which allows the operation to conclude quickly.
Instead of sweeping the tape head across the calibration track, the vertical position of each written calibration track is found by positioning the read head approximately straddling an edge of the calibration track. The vertical position of the particular calibration track is derived from the read head output which will be approximately half of the normal value due to the offset position. As the tape media moves up and down laterally the output will vary. Higher output levels indicate more head-track overlap while lower output levels indicate the opposite. The average read head output is used to determine the vertical position of the corresponding data tracks.